Waterproof covering materials formed by utilizing a water-absorbing or moisture-absorbing material are used in various fields. For example, in so-called conduction cables such as telecommunication cables and optical fiber communication cables, there must be no intrusion of water or moisture from the outside, and therefore, an especially elaborate waterproof covering is formed on the joins between cables. This waterproof covering is formed by applying a waterproof covering material to the join, winding a water-absorbing and moisture-absorbing material on the outer surface of the waterproof covering material, and coating a waterproof material such as a rubber on the water-absorbing and moisture-absorbing material. Water or moisture intruding into the covering layer through the outermost waterproof material is absorbed in the water-absorbing and moisture-absorbing material and is prevented from reaching the cable.
As the water-absorbing and moisture-absorbing material, there are used, for example, a split fiber of polyethylene glycol-impregnated polypropylene and a diethylene glycol-impregnated paper. However, these water-absorbing materials are difficult to handle because of their wettability, and furthermore, since a fluid is contained as the constituent, reproduction of the covering is very difficult.
As a means for solving these problems, a method has been proposed in which a powdery polymeric water absorber is incorporated and kneaded into a polyolefin at the extrusion step and a waterproofing property is imparted by subjecting the extrusion-shaped film to a drawing treatment and a splitting treatment. However, a polymeric water absorber generally has a poor resistance to heat, and if the polymeric water absorber is placed even at a melting temperature of a synthetic resin such as a polyolefin for a long time before extrusion shaping, the water-absorbing capacity is degraded, and moreover, the polymeric water absorber is substantially buried in the resin after extrusion shaping and the amount of the polymeric water absorber exposed to the surface is very small. Therefore, a satisfactory water-absorbing effect is not attained.
It is an object of the present invention to provide a water-absorbing material having a high water-absorbing efficiency and a good mechanical strength.